I have built a Theremax in a custom case and it sounds great and has a strong volume output.

Unfortunately I have a couple of issues. First I cannot get the volume to kill to zero when my hand is closest to the volume antenna. There is always a weak signal coming through.

Secondly, When moving my hand towards the volume antenna, the volume decreases gradually as it should (but does not kill to zero), but when moving my hand away again to increase the volume, the pitch "chirps" a few times with a couple of high pitch chirp noises. This only happens when moving my hand away from volume not towards.

Lastly, I have a strong volume output with negligible hiss, but I do pick up a strong radio signal. I can actually hear exactly what the radio presenters are saying through my amp.

The case is made of wood, would lining the inside of it with aluminium foil and rising the circuit off the bottom help with any of these issues?

Thanks for any help anyone can provide!

Wed Jun 04, 2014 5:04 am

PAiA-Scott

Joined: Sun Nov 09, 2008 2:49 pmPosts: 731

Re: Theremax Issues

Below is some information related to getting the best volume action -- following these details are some more specific tips and ideas.

It's important that wires don't lay on or near parts in the four oscillator circuits or the antennae cable ends, and, that the antennae mounts are about two inches from the metal bottom or other metal.

Skin and bones near these sensitive rf circuit nodes (in particular, the antennae) present a load that changes the frequency, and, the amplitude a bit, but metal and wires work even better and can put the oscillators out of tuning range or reduce their amplitude enough that the f-v section for the Volume CV (important for response with L3 adjustment) doesn't operate.

There's an image in the illustration supplements for reference and below are links to kit/panel and kit/case assemblies:

Having a metal base provides an anchor to the four oscillator circuit-sections and helps with tuning stability mostly, but will help with interference a bit.

Double-check to be sure the 100 and 68 pf capacitors in the pitch and volume oscillator sections are in their specified pairs and not mistakenly installed in 100/68 sets,

Connection with a line-level input amp vs a low-level (guitar or mic) input is desired for the least amount of background noise trouble.

Stopping with the pitch pair tuning at a lowest antiticpated "idling" pitch and not null minimizes interference from radio broadcasts which can work to teeter the pitch signal in and out of null, exaggerating this type of interference. Offsetting the initial tunings of the two pairs (ie set L1 another 1/8 or 1/16 turn in or out and readjusting L2).

If you can send photos of the completed unit, and, board top and bottom and board to panel wiring connections, I might be able to spot trouble and reply with advice.

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